Today, certain integrated circuit chips, such as microprocessors, are capable of operating at two different power supply levels. For instance, a chip may be able to operate using either a 3.3 volt or a 5 volt power supply. Furthermore, certain chips in the market today utilize different power supply levels for different portions of the chip. In other words, a portion of the chip may be powered by a power supply at one level (e.g., 5 volt supply), while another portion of the chip is powered by a power supply at a different level (e.g., 3.3 volt supply). Also a portion of the chip may be powered by a power supply at one level (e.g., 3.3 volt supply), while another portion of the chip is capable of operating at numerous power supply levels (e.g., 3.3 volts and 5 volts) at different times.
Integrated circuits are comprised of logic and circuit components. One common circuit or logic component on given integrated circuits is an input buffer. An input buffer allows data or signals off chip to be received as inputs for manipulation and use by the chip. Typically, an input buffer receives the offchip signal or data from a pin which is coupled to a circuit board or other communication means on which the signal is transferred. An input buffer is able to differentiate between a high and low input on the pin by comparing the input to a predetermined threshold value. This predetermined threshold value is referred to as the trip point of the input buffer. If the input data is greater than or has a voltage level above the trip point, then it is considered a high input. However, if the input data is at a level below the trip point, then it is considered a low input by the input buffer.
Prior art input buffers usually have fixed trip points. The trip point is usually fixed according to a ratioed value of its power supply level. Typically, a trip point for a 5 volt supply is set at 1.4 volts. However, the level of the trip point changes with changes in the power supply level. For example, an input buffer having a trip point of 1.4 volts for a 3.3 volt power supply, would have a trip point between 2.2 and 2.3 volts if powered by a 5 volt power supply.
A problem arises when the input buffer is coupled to receive an input from a transistor-transistor logic (TTL). The characteristic input from a TTL circuit typically has a high of approximately 2 volts and a low of approximately 0.8 volts. Prior art input buffers which are TTL compatible have a trip point of approximately 1.4 volts. Thus, in the prior art, an input buffer would be configured to have a 1.4 volt trip point for a fixed power supply voltage level. For example, an input buffer would be configured to operate with a trip point of 1.4 volts when powered by a 3.3 volt power supply. However, if this input buffer is powered by 5 volt power supply then the trip point would be 2.2 to 2.3 volts and would not be able to be tripped by a TTL data input. Therefore, what is needed is an input buffer which can operate at different power supply levels while recognizing both CMOS and TTL inputs.
As will be shown, the present invention comprises an input buffer which is capable of operating at different power supply levels. The input buffer of the present invention is capable of recognizing both CMOS and TTL inputs at the different power supply levels.